The present invention relates to heating systems in which a mixture of a fuel vapor and air are burned flamelessly on the surface of a catalyst. The invention relates particularly to the rapid and efficient starting of the combustion of the fuel when both the fuel and the catalyst are initially cold; i.e., at a temperature near room temperature or lower.
The catalytic oxidation of hydrocarbons, alcohols, and other chemicals at elevated temperatures has been practiced efficiently for many years in industrial processes, and more recently in such applications as small space heaters fueled by propane. A characteristic of such applications is that they operate steadily for relatively long periods, and are started from an initially cold state only infrequently. Thus, the catalytic space heater can be ignited with matches, and large industrial catalytic systems can be brought slowly up to operating temperature by auxiliary heaters. Such systems would be more conveniently started by a self igniting catalyst.
For certain applications which might be most advantageously served by catalytic heaters, however, the heating periods are very short and the system must be heated frequently from a cold start; hence very rapid and efficient initiation of combustion is mandatory, and an auxiliary heating system for starting combustion is unacceptable. One such application is the heating of shave foam or cosmetics dispensed from an aerosol can, where a few grams of material must be heated about 50.degree. to 80.degree. F in a period of 10 to 20 seconds, or less. In this case the heating system must be small and inexpensive, but capable of developing high heating intensities very quickly, and also capable of completely burning the fuel without production of noxious combustion products. It is also imperative that the system be highly efficient in converting and transferring the heating value of the fuel into the fluid which is to be heated.
Experience has shown, however, that suitable fuels, such as hydrocarbons and alcohols, when vaporized and mixed with air, will not start to burn spontaneously when brought into contact with catalysts of the types commonly used, unless the catalyst or the fuel, or both are preheated.
A further difficulty encountered in the use of conventional catalyst results from the fact that the catalytically active metal, for example platinum black, is most effective when supported upon highly porous base materials such as gamma alumina or silica gel, which are highly hydroscopic. Of the known catlyst compositions a mulled mixture of platinum and silica found in a small pellet may exhibit self ignition of methanol, but the reliability of such a pellet after exposure to high humidity is questionable. Commercially available pellets of alumina impregnated with small amounts, that is, less than 1%, of platinum family metal will not self ignite lower alcohols except at temperatures well above even high ambient temperatures. Thus, between heating periods when the system is cool, the catalyst support material absorbs moisture from the ambient air which greatly reduces the activity of the catalyst. This will completely block the spontaneous oxidation of preferred fuels such as methanol on the surface of the catalyst when both catalyst and fuel are initially cold. U.S. Pat. No. 3,191,659 to G. Weiss describes 30 mesh grains of alumina impregnated with 1/2 to 10% platinum by weight. The platinum concentration is then diminished by deposition on a fibrous sheet, but in any case my experience has shown that even a 10% concentration of platinum on alumina is ineffective to self ignite the lower alcohols at ambient temperatures after several days of exposure to an expectable range of natural humidity.
Accordingly it is the object of the present invention to provide a catalyst which will rapidly and reliably cause spontaneous ignition of lower alcohol fuels at hyper- and sub-normal temperature and humidity.